1. Field of the Invention
The present invention relates generally to medical devices and, more particularly, to sensors used for sensing physiological parameters of a patient.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring many such physiological characteristics. Such devices provide doctors and other healthcare personnel with the information they need to provide the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modem medicine.
One technique for monitoring certain physiological characteristics of a patient is commonly referred to as pulse oximetry, and the devices built based upon pulse oximetry techniques are commonly referred to as pulse oximeters. Pulse oximetry may be used to measure various blood flow characteristics, such as the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and/or the rate of blood pulsations corresponding to each heartbeat of a patient. In fact, the “pulse” in pulse oximetry refers to the time varying amount of arterial blood in the tissue during each cardiac cycle.
Pulse oximeters typically utilize a non-invasive sensor that transmits light through a patient's tissue and that photoelectrically detects the absorption and/or scattering of the transmitted light in such tissue. One or more of the above physiological characteristics may then be calculated based upon the amount of light absorbed or scattered. More specifically, the light passed through the tissue is typically selected to be of one or more wavelengths that may be absorbed or scattered by the blood in an amount correlative to the amount of the blood constituent present in the blood. The amount of light absorbed and/or scattered may then be used to estimate the amount of blood constituent in the tissue using various algorithms.
Pulse oximetry readings depend on pulsation of blood through the tissue. Thus, any event that interferes with the ability of the sensor to detect that pulsation can cause variability in these measurements. Motion artifacts occur when a patient's movements cause interference in the signal detected by the sensor. Motion artifacts can also occur in response to forces acting on the sensor. For example, a patient may be jostled by healthcare workers in emergency room settings. The type location, amount, or duration of force acting on a sensor will determine the nature of the motion artifact.
Generally, sensors are vulnerable to motion artifacts when the optical distance, or path length, orientation, or angle between a sensor's emitter and detector varies due to an undesired mechanical change in the conformation of the sensor while in use. The mechanical deformation of the sensor may be in the form of a compression of the sensor, causing a decrease in path length. Alternately, a sensor may flex or move in a manner that increases the distance between an emitter and detector, resulting in an increase in path length. In any case, variability in the optical path length due to motion can cause motion artifacts and obscure the desired pulse oximetry signal.